Oxide superconductors have been prepared by oxidation of a precursor alloy which contains the constituent metallic elements of the oxide superconductor and the matrix metal (typically, primarily silver). The matrix metal must itself be inert to oxidation or "noble" under the oxidation conditions employed during the process. The heat treatment of the composite is preferably carried out in two steps. A first heat treatment is carried out at relatively low temperatures in order to oxidize the component precursor elements into simple metal oxides or "suboxides". Subsequent heat treatments are then carried out at higher temperatures to convert the suboxide phases into the superconducting oxide phase(s). By "suboxide" as that term is used herein, it is meant simple, binary and/or ternary oxides of the component metals of the superconducting oxide.
Despite the relatively rapid diffusion of oxygen through the silver matrix phase, full oxidation of the metallic components into suboxides (in a first thermal treatment) can require extremely long times (for example, 600 hours). During heating at elevated temperatures, the mobilities of the precursor elements and their cationic forms are enhanced. The precursor elements diffuse into the surrounding silver metal, thereby impairing the chemical and physical integrity of the composite. This has the effect of interfering with the formation of oxide superconductor composites with good physical and electrical properties.
By "precursor elements", as that term is used herein, it is meant the individual metallic elements of the precursor alloy or their cationic forms. Typically, the precursor elements diffuse as neutral species; however, cations are also expected to contribute, in varying degrees, to the mobility of the precursor elements. Copper has the highest mobility of the constituent precursor elements, but also barium in the yttrium-barium-copper-oxygen (YBCO) system, bismuth and/or lead in the bismuth(lead)-strontium-calcium-copper-oxide (BSCCO) system and thallium and/or lead in the thallium(lead)-strontium-calcium-copper-oxide (TlSCCO) system are known to diffuse into the silver matrix. It is expected that given sufficient time and appropriate reaction conditions, other elements will also have measurable mobilities in silver at a level sufficient to impair composite mechanical and electrical properties.
In particular, the ability of precursor elements to segregate during thermal treatment has made it difficult to prepare multifilamentary wires having high filament counts. By "multifilamentary wires", as that term is used herein, it is meant wire, rods, tapes and the like, containing oxide superconductor filaments within a matrix metal, where the filaments run axially parallel to one another along the length of the wire, the "longest dimension". By "high filament count", as that term is used herein, it is meant filament densities of greater than 10,000 filaments/cm.sup.2 as determined for a cross-section transverse to the longest dimension. At high filament densities, even the slightest segregation of precursor elements results in the coalescence of individual filaments and the deterioration of wire properties.
High oxygen pressure has been used in the internal oxidation of Sn--Ag alloys. Tanaka et al. in U.S. Pat. No. 5,078,810 (hereinafter, "Tanaka") observed that high pressure oxidation eliminated scale formation of tin oxides on the outer surface of the silver composite due to the diffusion of tin to the surface. Tanaka addresses the problem of tin migration to the composite surface and not the segregation of tin within the composite. Indeed, segregation such as that observed for complex oxide superconductor composites can not occur in the simple tin oxide-silver composites disclosed by Tanaka.
It is an object of the present invention to provide a low temperature oxidation process for preparation of a metal oxide which minimizes diffusion and segregation of the precursor elements into the silver phase without unfavorably affecting the oxidation time.
It is a further object of the invention to provide a low temperature oxidation process for preparation of an oxide superconductor which minimizes diffusion and segregation of the precursor elements into the silver phase without unfavorably affecting the oxidation time. The object of the present invention provides an oxide superconductor composite with improved properties, such as high critical current density.
It is a further object of the invention to provide an oxide superconductor multifilamentary composite wire characterized by a highly unsegregated microstructure and a high filament count.